The present invention relates to a liquid crystal display panel, and more particularly, to a liquid crystal display panel in which an oxide thin film transistor using an oxide semiconductor for a semiconductor layer is provided in the periphery of a pixel portion of the liquid crystal display panel, as a thin film transistor (TFT) to control pixel electrodes.
For example, liquid crystal display devices using liquid crystal display panels are used as display devices for various portable information devices such as mobile phones, portable information terminals (PDAs), digital cameras, and multimedia players, or for display devices of television and personal computers.
In the field of such liquid crystal display panels, liquid crystal display panels using oxide thin film transistors (oxide TFTs) have attracted attention as a means for achieving high definition.
Compared to amorphous silicon TFT (a-SiTFT), oxide TFT is a thin film transistor with high mobility and performance by substantially the same process. However, it is important to control the threshold voltage and to prevent the time-dependent degradation of TFT characteristics.
Further, there is a method called photo alignment to apply an alignment treatment to an alignment film used for the liquid crystal display panel. In other words, the photo alignment is a method for providing an alignment control function to the alignment film. By using the photo aliment method, for example, it is possible to suppress light leakage and increase the contrast of a display image.
However, a problem arises when the photo alignment is applied in the liquid crystal display panel using an oxide TFT. More specifically, when ultraviolet light is irradiated onto the oxide TFT, defects are generated in the semiconductor layer, which results in a shift of the threshold voltage. In addition, there is also a problem that the defects accelerate the time-dependent degradation of TFT characteristics.
Such threshold voltage shift and time-dependent degradation of TFT characteristics may be a problem particularly in the oxide TFT applied to the peripheral circuit to control the pixel electrodes, rather than in the oxide TFT applied to the pixel electrodes. In the peripheral circuit, a small threshold voltage shift and time-dependent degradation of TFT characteristics may cause a problem in the normal circuit operation, namely, in the normal image display.
For example, an oxide TFT of In, Ga, Zn, and O, (for example, the band gap of IGZO is about 3 to 3.2 eV), absorbs ultraviolet light. For example, when a mercury lamp is used for photo alignment, ultraviolet light with wavelengths of 254 nm, 313 nm, 365 nm, 405 nm, 436 nm, and the like are irradiated onto the IGZO layer to form defect levels in the semiconductor layer. At this time, if the defect levels are of donor type, the threshold voltage is depleted. In addition, increasing such defect levels accelerates the time-dependent degradation of TFT characteristics.
For example, in the oxide TFT configuring a peripheral circuit such as a gate scanning circuit or a selector circuit, if the threshold voltage shifts in the negative direction (and is depleted), a problem arises that the peripheral circuit fails to perform the off operation and would not operate normally.
Japanese Patent Application Laid-Open No. 2012-181447 describes an invention in which an ultraviolet absorbing layer is formed in a counter substrate, to absorb ultraviolet light causing degradation of an alignment film that is subjected to photo alignment treatment in a TFT substrate, by the ultraviolet absorbing layer, in the process of curing the resin with ultraviolet light to bond the counter substrate and the TFT substrate, in order to prevent the alignment film from being damaged by the ultraviolet light.